1. Field of the Invention
The present invention relates to an apparatus for and a method of evaluating semiconductor material. In particular, it relates to an apparatus for and a method of evaluating processes such as an ion implantation process conducted on semiconductor substrates through inspection of crystal defects caused in the semiconductor substrates due to the processes.
2. Description of Related Art
To form integrated circuits on a semiconductor wafer, the wafer is passed through a series of processes including an ion implantation process that implants ions, charged atoms or charged molecules in the wafer. During ion implantation, the implanted ions collide with crystal lattices in the wafer, causing point defects such as interstitial-atoms and vacancies. There are related arts for inspecting crystal defects in the wafer before and after ion implantation and evaluating various characteristics of the wafer according to a result of the inspection.
One of the such related arts irradiates a pump beam on an ion-implanted semiconductor wafer, the pump beam having energy equal to or larger than a bandgap width of the wafer. Some photons in the pump beam are absorbed by the wafer and generate photocarriers each consisting of a pair of a hole and an electron in the wafer. The related art irradiates a probe beam on the wafer, catches a reflected probe beam from the wafer, and from the reflected probe beam, reads a recombination process of the photocarriers in the wafer. The recombination process of excess carriers including the photocarriers depends on a crystallographic state in the wafer, and therefore, reading the recombination process results in reading the quantity of impurities implanted in the wafer, the depth of a preamorphous region in the wafer, and an after-annealing junction depth in the wafer.
The excess carriers in the semiconductor wafer caused by the pump beam recombine at a certain time constant. Due to the pump beam, point defects in the wafer move or annihilate to change the spatial distributions and concentration profile of the point defects during measurement and affect the recombination process of the excess carriers. Also, the intensity of the reflected probe beam varies depending on a measuring time, making it difficult to correctly measure the characteristics of the wafer. This problem becomes serious when a modulation frequency of kilohertz order is applied to the pump beam that pumps photocarriers in the wafer.
When the surface of the semiconductor wafer has a film of, for example, chemical oxide with much trapped charge, the film will change the intensity and phase of a reflected pump beam by several tens of percent.